During Leg 200, we completed drilling operations for two distinct projects: (1) the Hawaii-2 Observatory (H2O), where we established a cased reentry borehole at Site 1224 and sampled the upper oceanic crust, and (2) the Nuuanu Landslide, where we recovered landslide deposits at Site 1223 that were derived from the Hawaiian Islands. The primary focus of the cruise was drilling at H2O, with Nuuanu Landslide drilling only being added to the Ocean Drilling Program (ODP) operations schedule about three months prior to the cruise and consuming only two days of operations. Below, we describe each project separately, focusing first on the H2O results and then the Nuuanu Landslide results.
The long-term H2O site satisfies three scientific objectives of crustal drilling: (1) it is located in one of the high-priority regions for the Ocean Seismic Network; (2) its proximity to the Hawaii-2 cable and the H2O junction box makes it a unique site for real-time, continuous monitoring of geophysical and geochemical experiments in the crust; and (3) it is on fast-spreading Pacific crust (7.1 cm/yr half-rate), which represents one end-member for models of crustal generation and evolution and crust/mantle interaction. The H2O junction box is in the eastern Pacific at 27°52.916'N, 141°59.504'W at a water depth of 4979 m, roughly halfway between California and Hawaii. The primary goal of the leg was to drill a suitable hole for a borehole seismometer that will be installed later. This was accomplished in Hole 1224D, where we installed a reentry cone and cemented casing 30 m into basaltic basement 1.48 km northeast of the H2O junction box. Above basement there was 28–30 m of soft, red clay. The cased basement interval, in which the instrument will be installed, consisted of two well-consolidated massive basalt flows. We also drilled a second single-bit hole, which was cored and logged, within 20 m of the first to a depth of 145 m into basement. The second hole was left with a free-fall funnel so that it also could be reentered using the wireline reentry technology to carry out other borehole experiments at the site. In addition to a suite of shipboard physical and chemical analyses that can be used to characterize the crust surrounding the observatory, we also conducted microbiological analyses of the recovered sediments and basalts. As a general trend, bacterial population numbers decreased with increasing depth, though the amount of metabolically active bacteria remained remarkably high at 41%–62% of the total cell counts. The successful cultivation of oxidizing bacteria and the microscopic indication of further microbial structures within a basaltic rock confirm the presence and even activity of microbial life not only in deep marine sediments, but also in the Paleogene oceanic crust from the North Pacific.
During Leg 200, we also cored the Nuuanu Landslide site on the Hawaiian Arch, ~300 km northeast of Honolulu. The upper 100 m of sediment at this site was thought to contain a record of the Nuuanu Landslide, a catastrophic event or series of events that removed ~40% (3000 to 4000 km3) of the Koolau Volcano on the island of Oahu. We recovered several lithologic units that were transported to the site by a number of distinct landslide events. The origin of the deposits, as indicated by petrographic inspection and geochemistry, is the Hawaiian Islands. Two pyroclastic events, similar to the 1980 Mount Saint Helens' eruption but an order of magnitude larger, occurred on Koolau at ~2 Ma. These events may correlate with the collapse of the flank of the volcano and the formation of the Nuuanu debris field. The turbidites and pyroclastic material are of similar age to the Nuuanu Landslide (1.8–2.4 Ma) and are more than 38 m thick at Site 1223, >300 km from Oahu. We did not core to the bottom of the Nuuanu-related sequence. Thus the Nuuanu-related deposits may be thicker, and additional landslide events may have occurred.